Is it permissible for a man to marry the daughter of his paternal uncle who is married to my maternal grandmother, bearing in mind that this daughter is from another wife?
Moreover, there is no foster relation between this daughter whom I want to marry and my maternal grandmother. I doubted the permissibility of marrying her because my maternal grandmother is one of her father's wives. The children of my grandmother are my maternal uncles. Moreover, she is a sister of my uncles.
If the reality is as you mentioned that your paternal uncle is married to another wife other than your maternal grandmother and that your paternal uncle's daughter is not the daughter of your maternal grandmother and that there is no foster relation between her and your grandmother, you will be permitted to marry her. The basic rule is permissibility as long as there is no evidence that forbids you from marrying her. Moreover, your maternal grandmother being a wife of the father of this girl does not affect marrying her as long as your grandmother is not her mother or foster mother. (Part No. 18; Page No. 344) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.